The present invention relates generally to the in situ measurement of the bulk electrical properties of various substances, often fluid mixtures, and the interpretation of such direct electrical measurements to produce an indirect or inferred measurement of the composition of a given substance or mixture based upon the change in electrical properties that occurs as the relative percentages of the components in the mixture vary. In some situations the invention also has application to solid substances having a surface that can be interrogated by placing the surface of a suitably configured probe against or in close proximity to the substance surface.
The electrical permittivity of a nominally homogeneous mixture depends upon the volumetric ratio of the constituent materials and upon the permittivity of the individual components. Microwave instruments exploit this fact to analyze the properties of substances or the composition of mixtures by measuring and analyzing various attributes of a microwave signal or set of signals that depend directly upon the permittivity of the substance or mixture. For example, instruments in a variety of configurations are available which measure the attenuation or phase shift of signals that are transmitted through an unknown mixture. The material sample may be placed between transmitting and receiving antennas as described in Swanson, U.S. Pat. No. 4,812,739. Alternatively, the sample may be loaded into a coaxial or waveguide structure that supports the propagation of the wave or waves according to the inventions contained in Jean et al., U.S. Pat. No. 5,455,516; Scott et al. U.S. Pat. No. 4,862,060; or De et al. U.S. Pat. No. 4,902,961.
Likewise, prior-art reflection sensors are available which rely on measuring the amplitude and phase of the reflection coefficient at the interface between a probe element and the mixture surface. For example, see xe2x80x9cA Novel Numerical Technique for Dielectric Measurement of Generally Lossy Dielectricsxe2x80x9d by Ganchev, Bakhtiari, and Zorghi, IEEE Transactions on Instrumentation and Measurement, Vol. 41, No 3, June 1992. However, even the most sensitive reflection sensors cannot reliably measure the extremely small electrical differences that are associated with many important applications, such as the measurement of steam quality.
Shearer et al. teach a microwave absorption technique for analyzing gases in U.S. Pat. No. 5,507,173. This analyzer employs parallel microwave beams, but the separate beams pass through independent measurement cells and an elaborate arrangement of attenuators and signal splitters is needed to determine the difference in microwave absorption between a one cell containing a reference gas and another containing the gas under test. The analyzer operates at a single carefully controlled frequency selected to correspond to an absorption line of the gas being analyzed.
Carullo, Ferrero and Parvis in xe2x80x9cA Microwave Interferometer System for Humidity Measurementxe2x80x9d describe an interferometer technique for the measurement of humidity. This interferometer falls short in two important respects. First, the dynamic range of the interferometer method described by Carullo is severely limited. Secondly, Carullo describes a phase measurement being made on interfering signals that are at a constant frequency. As a consequence, the interferometer is severely limited in sensitivity and accuracy. Also, the Carullo interferometer does not allow for the measurement of the loss factor of the material.
There is a significant need for a microwave-based sensor that has sufficient precision to reliably monitor the composition of mixtures of gases, while having sufficient dynamic range to address applications where the mixtures contain large variations in moisture and the process undergoes large swings in pressure and temperature.
In addition, there is a need for a microwave sensor that can maintain a sensitive measurement as the probe element is subjected to wear or corrosion in harsh environment of the measurement zone. Additional needs include that the probe be insensitive to stray reflections and other signal artifacts that can render prior art sensor inoperative and that the probe requires very low signal power to operate. The ability to operate at very low power levels is desired to reduce the sensor cost and mitigate operational problems in satisfying FCC rules.
As an example of the need for an improved microwave sensor, consider the application of measuring the composition of a gas mixture such as encountered in the measurement of steam quality. Gases have dielectric constants very near that of free space. For example, dry steam at 110 degrees Celsius has a relative dielectric constant of 1.0126 as reported in the Handbook of Chemistry and Physics 63rd Edition, CRC Press, Inc., 1983. Theoretical computations predict that the relative dielectric constant for 50% quality steam will increase to only 1.081. This change in dielectric constant is 6.75% for a 50% change in steam quality. It will be clear in the descriptions that follow that the present invention can easily distinguish such small changes.
The present invention achieves technical advantages as a microwave-based sensor having improved sensitivity over prior art microwave-based sensors. The sensor is rugged in construction and low in cost to produce.
One preferred embodiment of the invention is a probe-type sensing element which is insertable into a vessel or pipe and is suitable for monitoring changes in the electrical properties of steam. A sensor with improved sensitivity is required for steam quality measurement because of the relatively small change in permittivity of wet steam over a quality range as large as even from 50 to 100%. As will be evident from the discussions that follow, other configurations for the sensor that are suitable for a wide range of applications are also contemplated within the scope of the invention and examples will be given.
The sensor accomplishes a measurement by varying the frequency of the microwave excitation signal and observing when a minimum (xe2x80x9cnullxe2x80x9d) is detected for the vector sum of two output signals. The two signals travel unequal electrical distances. The vector summation will be a minimum (null) whenever the electrical traveled distances differ by a half wavelength, or an odd integral multiple of a half wavelength. It should be appreciated by those skilled in the art that the same general effect can be obtained by inverting the signal in one of the signal paths, such that the signals will produce a minimum output (null) for those frequencies for which the paths differ in length by a full wavelength or an integral multiple of a full wavelength. The electrical distance of travel is dependent upon the dielectric properties of the material under test. As the electrical permittivity (or dielectric constant, as it is commonly known) of the material mixture responds to changing amounts of its electrically different components, then the frequency required to make the electrical length difference equal to a half wavelength will also change. Determining this null frequency therefore represents a direct measurement of the dielectric constant and hence the relative composition of the mixture.
Consider some specifics of the design, for example, of a sensor for the measurement of steam quality. Such a sensor may employ a probe that has signal paths that differ by 1.5 cm in physical length. In vacuum, this path difference corresponds to a half wavelength at a frequency of approximately 10 GHz. For dry steam according to the conditions specified above, the frequency shifts to a value of 9.937589 GHz. For 50% quality steam, a frequency of 9.618353 GHz is observed, a frequency difference of more than 319 MHz. Microwave circuits are readily available which are stable to within a few parts per million and frequency measurements are easily accomplished to a similar precision. If we consider a frequency measurement accuracy of only 100 parts per million, then the 50% steam quality range can be measured to a precision of +/xe2x88x920.0156% of steam quality. Certainly, there are other factors, such as temperature and pressure, that will affect the steam quality measurement, but the sensor itself clearly has sufficient precision for the task.
The null frequency value depends primarily upon the real part of the permittivity. As the signals from the two paths are combined so as to produce the null by destructive interference, the depth of the null will depend upon the amount of attenuation that the waves have experienced. Observing the depth of the null thus provides one measure of the imaginary part of the permittivity or loss factor. The total amplitude of the reflection for an off-null condition will also be indicative of the loss factor encountered by the signals. The overall length of the signal paths will influence the sensitivity of the loss factor measurement. If it is not required to measure loss factor, then only one signal path equal to a half-wavelength distance need be inserted in the process stream. It is also possible to expose only one signal path along its full length or any portion of its full length to the process while the second signal path functions simply as a fixed reference. In most applications it will be beneficial to have two signal paths inserted into the process stream so as to take advantage of common mode rejection of the effects of external parameters that are unrelated to the composition measurement.
One alternative configuration for a microwave sensor according to the teaching of the present invention is to embed the transmission lines in a dielectric material along the walls of the pipe or vessel so as to not have a probe element protruding into the process stream or vessel. Such a configuration will have less sensitivity than a probe-type sensing element, but there are many applications that can be addressed with more than enough precision owing to the inherent advantages of the invention.
Yet another configuration is to embed the transmission lines in a dielectric material and still configure the sensor as a probe element. Such a configuration would be suitable for a portable version of the instrument for applications such as the sampled measurement of the percent of fat in processed or ground meat.
Yet another configuration consistent with the teaching of the present invention is to transmit the microwave energy through the process material and to have either two transmitting elements or two receiving elements and locate them in a position such that parallel paths differing in length are formed. For this configuration, the transmitting and receiving elements are embedded in the walls of a metal pipe or vessel using suitable microwave transparent windows, or for the case of a plastic pipe or vessel, the transmitting and receiving elements are mounted on the outside of the pipe. For this configuration care must be taken to prevent the occurrence of multiple signal paths, which would potentially introduce false signals and readings.
Yet another configuration according to the present invention is to cause the process material to flow through, or to statically fill, the through arms of a rectangular waveguide hybrid-tee. An abrupt impedance mismatch is placed in each arm at distances from the center of the hybrid junction differing by a half wavelength according to the teaching of the invention described above. The excitation signal is supplied to either the E-plane or H-plane arm of the tee. The excitation signal is equally divided into the through arms of the tee and is reflected from the abrupt impedance mismatch back toward the hybrid junction. The vector summation signal will then appear at either the H-plane or E-plane arm, respectively.